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Amid the buzz over passive immunotherapy as a potential treatment for Alzheimer disease, the prevention chorus has swelled a tad. Researchers led by David Morgan of the University of South Florida, Tampa, have used an AD mouse model to distinguish between two explanations for how amyloid accumulates in the brain. Their findings, reported in the 15 April Journal of Neuroscience, offer strong support for a prevailing hypothesis—that plaques grow from amyloid-β “seeds” that sneak onto the scene when Aβ production exceeds clearance by small amounts over time. An unfortunate therapeutic implication of this model is that reducing Aβ production or increasing its clearance may alleviate future Aβ deposition but would not remove existing seeds. However, the new study offered a silver lining. “A little bit of prevention of Aβ deposition very early has very, very significant benefits later on. It’s going to suppress the entire disorder back in time dramatically,” Morgan told ARF. He estimated that scaling back Aβ deposition by just a year or two in its early stages could delay the appearance of dementia by five to 10 years.

According to a leading theory, which the new paper calls the “accumulation hypothesis,” the brain’s amyloid load reflects the cumulative effect of Aβ production outweighing clearance to a slight extent. With time, these small amounts of extra Aβ huddle together and seed the comparatively quicker outgrowth of fibrillar Aβ that researchers can see under a microscope. However, amyloid buildup could also be explained by an alternative “equilibrium hypothesis,” which suggests that steady-state rates of Aβ production and clearance change with age and that this imbalance determines the extent of amyloid deposition.

Support for the latter notion comes from studies showing that older people have reduced levels of Aβ-degrading proteases in AD-affected brain areas (Iwata et al., 2002; Caccamo et al., 2005). In addition, several reports have documented the rapid reappearance of Aβ plaques after amyloid load had been dramatically reduced by brain injections of LPS (Herber et al., 2004) or anti-Aβ antibody (Oddo et al., 2004 and ARF related news story). In both cases, Aβ levels “returned back in a matter of weeks to the levels they were at previously—in spite of the fact that they had been building over a period of months,” Morgan said. These observations led his team to consider that the extent of amyloid deposition might reflect disproportion between production and clearance—whether induced by age or treatment—and that Aβ levels shift back when rates of these processes are restored to equilibrium.

To distinguish the two hypotheses, first author Rachel Karlnoski and colleagues suppressed amyloid deposition in Tg2576 mice by systemic treatment with a humanized, monoclonal antibody against Aβ’s C-terminal end. (Originally developed at Rinat Neuroscience, which has since been acquired by Pfizer, this antibody [PF-04360365] is currently in Phase 2 trials for AD.) The researchers began treating the mice when they were eight months old, about a month before amyloid deposition typically begins in this strain, and continued weekly injections for six months. At that point, they stopped treatment and used immunohistochemistry and ELISA to monitor what happened to amyloid load over the next three months. If the accumulation hypothesis is correct, they reasoned, amyloid load in the antibody-treated mice would begin increasing at the same rate as in the control mice, but never catch up in absolute levels, starting from the point at which treatment was stopped. The equilibrium hypothesis, on the other hand, predicts that amyloid deposition would occur at a faster rate in the antibody-treated mice once Aβ suppression was lifted, such that their amyloid load would eventually catch up to that of the control mice.

In this study, the accumulation hypothesis came out as the hands-down winner. By histology, amyloid load in the frontal cortex dropped sharply after six months of immunotherapy, and Aβ levels remained much lower in the treated mice three months after immunizations were stopped. ELISA measurements of Aβ40 and Aβ42 backed these findings. The trends also held when the researchers stained frontal cortex for several microglial activation markers corresponding to early, intermediate, and mature stages of Aβ deposition.

All told, the findings are consistent with the idea that “there is some crystal of Aβ somewhere that is driving deposition of the Aβ peptides,” Morgan said. “Even if we whittle away at the ends…it can rapidly regrow. The rate limitation is the number of ‘seeds,’ and our treatments are not clearing the seeds out of the brain.”

Previous work in AD transgenic mice has shown that anti-Aβ immunization is most effective during the early stages of Aβ deposition and does not work well once amyloidosis is well underway (see, e.g., Das et al., 2001; Levites et al., 2006; and ARF related news story). An important piece of data from the new paper “is that even a window treatment at the right stage can delay future accumulation of pathology,” wrote Yona Levites, first author of one of the previous studies, in an e-mail to ARF. Levites, a former postdoctoral fellow in Todd Golde’s lab at Mayo Clinic in Jacksonville, Florida.

Ongoing work by Golde and colleagues aims to tease out at which stage one can achieve maximal efficacy with amyloid-reducing drugs such as γ-secretase inhibitors (see ARF related news story). In the meantime, Morgan’s group plans to address another aspect of the accumulation hypothesis—the idea that after initial seeding, amyloid deposition proceeds apace for a while but eventually reaches a plateau. To examine this, the researchers will essentially repeat the current study, but instead of seeing how amyloid load changes three months after suppression of treatment, they will wait six to eight months—“to determine whether there is some upper limit beyond which the animals cannot deposit amyloid,” Morgan said.—Esther Landhuis

Comments

This is an intriguing and nice study. It tests the hypothesis whether Aβ plaques are in direct equilibrium with soluble Aβ. The results point to an accumulation model, and the given explanation for discrepancies with other well-reproducible observations is evident. It seems appropriate to speculate that plaque formation in vivo is a two-step process that involves a slow buildup of an Aβ seed (which contains oligomeric Aβ forms) and is followed by a rather fast and reversible (e.g., through immunotherapy) second step of growth to a histologically detectable and well-defined aggregate. Assuming that this is correct, it raises further questions that are extremely interesting:

1. Why is such a small seed stable enough not to be dissolved upon treatment with antibodies?
2. Why do at least some antibodies permanently block the conversion from a seed to a mature plaque (e.g., Meyer-Luehmann et al., 2006?
3. Is it possible to isolate such “core seeds” and will they still function as such when transferred to another host animal?
4. If so, what other components are contained in such a “core seed”?